Tungsten filament lamps have been heavily relied on in the past for navigational signal lighting. Besides having low efficiencies, the filament in such lamps is very brittle and therefore susceptible to shock and vibration. This results in premature lamp failure. Also, in general, they have short life of about 500 hours.
Modern light sources, more particularly arc discharge sources, have been or are being developed for navigational signal lighting applications because of the many advantages offered by these light sources. It is well known that an arc discharge source generally provides better efficacy and longer life than its tungsten filament lamp counterpart. Since the electrodes are heavier than the filament, the lamp may be more rugged and less susceptible to shock and vibration.
In an arc discharge lamp, the length and width of the arc are design variables to a large extent. In a tungsten filament lamp, the length and width of the filament are for the most part determined by the lamp wattage. Thus, there is greater flexibility in the choice of optical characteristics of the light source with arc discharge lamps than with comparable tungsten filament lamps.
The principal object of a navigational signal light is to emit as much light flux as possible from a reliable light source and direct the light into the plane of the horizon. The light may be collected into one or more narrow beams which are mechanically rotated, or it may be radiated in all horizontal directions simultaneously. There are basically two types of rotating beams or beacons. In the first type, a reflector or other means of concentrating the light is used with the lamp. The entire optical system is rotated. This method generally produces a single beam; all of the emitted light is swept through 360 degrees. For an example of this first type of beacon and an arc discharge lamp for use therewith, see U.S. Pat. No. 4,847,530 which issued to English et al and which is assigned to the same Assignee as the present application. This patent describes an arc discharge lamp which, in one specific example, is rated at 175 watts.
In the second type, a rotating screen surrounds a stationary lamp. The screen contains multiple lenses or other means for concentrating light. The rotating screen generally produces multiple rotating beams, one beam associated with each lens or sector subtended by a lens. The emitted light within any sector is formed into a pencil beam and swept only within that sector. It is this type of beacon and, more particularly, the light source associated therewith, which is the subject of this disclosure.
For an example of this second type of beacon and an arc discharge lamp for use therewith, see U.S. Pat. No. 4,864,180 which issued to English et al and which is assigned to the same Assignee as the present application. This patent describes a metal-halide arc discharge lamp which, in one specific example, is rated at 45 watts.
Although the arc discharge lamps in the above-described patents are quite suitable for various navigational lighting applications, they would tend to generate too much heat if enclosed within the relatively small beacon enclosure of a typical lighted buoy.
It would be an advancement of the art if a miniature, low-wattage arc discharge lamp could be provided which is suitable for use in a navigational signal light, such as a lighted buoy.
Fluorescent lamps are well known in the art and are used for a variety of lighting applications. Standard fluorescent lamps are characterized as low-pressure arc discharge lamps and may include an envelope whose internal surface is coated with phosphor. An electrode structure is located at each end of the envelope. The envelope contains a quantity of an ionizable medium, such as mercury and a fill gas at a low pressure, for example, in the order of 1 to 5 torr.
When a voltage is applied across the electrodes, electrons will be emitted, ionizing the gas inside the envelope. The resultant ionization and recombination of ions and electrons produce primarily short wavelength ultraviolet radiation of 253.7 nanometers which is converted by means of the phosphor into radiation of a longer wavelength and a spectral distribution (depending on the phosphor material used) in the near ultraviolet or in the visible part of the spectrum.
One of the most important aspects of discharge lamp design is to ensure that as much of the input power as possible is directed into transitions producing the desired wavelengths (e.g., 253.7 nanometers). The problems in doing this efficiently multipy as the size of the source decreases.